[Qemu-devel] [RFC PATCH] migration/postcopy: skip compression when postcopy is active

[Wei Yang <richardw.yang@linux.intel.com>]

Now postcopy is not compatible with compression. And we disable setting
these two capability at the same time. While we can still leverage
compress before postcopy is active, for example at the bulk stage.

This patch skips compression when postcopy is active instead of
forbidding setting these capability at the same time.

Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
---
 migration/migration.c | 11 -----------
 migration/ram.c       | 10 ++++++++++
 2 files changed, 10 insertions(+), 11 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index 5a496addbd..33c373033d 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -995,17 +995,6 @@ static bool migrate_caps_check(bool *cap_list,
 #endif
 
     if (cap_list[MIGRATION_CAPABILITY_POSTCOPY_RAM]) {
-        if (cap_list[MIGRATION_CAPABILITY_COMPRESS]) {
-            /* The decompression threads asynchronously write into RAM
-             * rather than use the atomic copies needed to avoid
-             * userfaulting.  It should be possible to fix the decompression
-             * threads for compatibility in future.
-             */
-            error_setg(errp, "Postcopy is not currently compatible "
-                       "with compression");
-            return false;
-        }
-
         /* This check is reasonably expensive, so only when it's being
          * set the first time, also it's only the destination that needs
          * special support.
diff --git a/migration/ram.c b/migration/ram.c
index da12774a24..a0d3bc60b2 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -2384,6 +2384,16 @@ static bool save_page_use_compression(RAMState *rs)
         return false;
     }
 
+    /*
+     * The decompression threads asynchronously write into RAM
+     * rather than use the atomic copies needed to avoid
+     * userfaulting.  It should be possible to fix the decompression
+     * threads for compatibility in future.
+     */
+    if (migration_in_postcopy()) {
+        return false;
+    }
+
     /*
      * If xbzrle is on, stop using the data compression after first
      * round of migration even if compression is enabled. In theory,
-- 
2.17.1